Agriculture in an even more sunburnt country - Four Degrees of Global Warming - page 97

et al., 2010; Jones and Page, 2001; Sanders et al., 2010). This will further reduce
the production levels and alter commodities grown in these regions, with the
impacts of climate change expected to interact with ongoing changes in institu-
tional arrangements. Increasing water scarcity in key irrigation areas such as the
Murray-Darling Basin region is predicted to significantly reduce production rates
by 2030 (Sanders et al., 2010) well before a 4°C temperature increase occurs.
Pests, diseases and weeds
Climate change is likely to alter the range, severity and species of pests, diseases
and weeds. However, the specific changes that might occur are uncertain, and few
studies have extrapolated existing knowledge to changes associated with a 4°C
world. There may be a range of possible outcomes (Chakraborty et al . , 1998). For
example, with the wheat disease stripe rust ( Puccinia striiformis ), a temperature
rise may increase the amount of this rust but not necessarily mean additional
yield losses with the changes in impact of this rust varying regionally, with
management and with cultivar. The fungal disease Take-all ( Gaeumannomyces
graminis ) can cause major crop losses when there are extended periods of high soil
moisture. Its severity may be reduced if there is an increase in rainfall variability
and drier winters as suggested by the climate scenarios covered here. Septoria
blotch ( Septoria tritici ) incidence is affected by sowing time and rainfall at
heading. The current scenarios of reduced rainfall over southern Australia (less
severe infection) but increased temperature (more severe infection) result in an
uncertain outcome. Viral diseases such as Barley Yellow Dwarf, which rely on
transfer by aphids, may increase with warmer winter temperatures with potential
additional risks via the positive effects of CO 2 concentration on aphid popula-
tions (e.g. Newman et al . , 2003). Climate change may also affect the balance
between soil-based pathogens like Fusarium graminearum and their antagonists
such as Trichoderma , but again, outcomes are uncertain. In some instances,
changes in farm management to reduce evaporative losses (e.g. zero tillage) may
also interact with disease risk. In recent years the increase in the prevalence
of Rhizoctonia has anecdotally been linked to wetter than normal conditions
combined with zero tillage practices.
The impact of insects on Australian crops in a 4°C world is currently uncertain.
Some experiments indicate that climate change and increases in atmospheric
CO 2 may increase insect damage. This can occur as a result of compensatory
feeding when elevated CO 2 reduces leaf nitrogen concentrations (i.e. the insect
has to eat more to maintain nitrogen intake; Lincoln et al . , 1986). In other cases,
increased CO 2 results in increased concentrations of plant defensive compounds
such as condensed tannins, and this plus lowered nitrogen concentrations with
elevated CO 2 can reduce insect herbivore weight gains, increase mortality and
lower fecundity (e.g. Gao et al . , 2008).
There are also concerns that a range of weed species (especially summer-
growing C 4 weeds) will increase their competitive advantage under elevated
CO 2 and higher temperatures (e.g. IPCC, 2007b). This may become even more